1rfkill - RF kill switch support
2===============================
3 41. Introduction
52. Implementation details
63. Kernel API
74. Userspace support
8 9 101. Introduction
11 12The rfkill subsystem provides a generic interface to disabling any radio
13transmitter in the system. When a transmitter is blocked, it shall not
14radiate any power.
15 16The subsystem also provides the ability to react on button presses and
17disable all transmitters of a certain type (or all). This is intended for
18situations where transmitters need to be turned off, for example on
19aircraft.
20 21The rfkill subsystem has a concept of "hard" and "soft" block, which
22differ little in their meaning (block == transmitters off) but rather in
23whether they can be changed or not:
24 - hard block: read-only radio block that cannot be overriden by software
25 - soft block: writable radio block (need not be readable) that is set by
26 the system software.
27 28 292. Implementation details
30 31The rfkill subsystem is composed of three main components:
32 * the rfkill core,
33 * the deprecated rfkill-input module (an input layer handler, being
34 replaced by userspace policy code) and
35 * the rfkill drivers.
36 37The rfkill core provides API for kernel drivers to register their radio
38transmitter with the kernel, methods for turning it on and off and, letting
39the system know about hardware-disabled states that may be implemented on
40the device.
41 42The rfkill core code also notifies userspace of state changes, and provides
43ways for userspace to query the current states. See the "Userspace support"
44section below.
45 46When the device is hard-blocked (either by a call to rfkill_set_hw_state()
47or from query_hw_block) set_block() will be invoked for additional software
48block, but drivers can ignore the method call since they can use the return
49value of the function rfkill_set_hw_state() to sync the software state
50instead of keeping track of calls to set_block(). In fact, drivers should
51use the return value of rfkill_set_hw_state() unless the hardware actually
52keeps track of soft and hard block separately.
53 54 553. Kernel API
56 57 58Drivers for radio transmitters normally implement an rfkill driver.
59 60Platform drivers might implement input devices if the rfkill button is just
61that, a button. If that button influences the hardware then you need to
62implement an rfkill driver instead. This also applies if the platform provides
63a way to turn on/off the transmitter(s).
64 65For some platforms, it is possible that the hardware state changes during
66suspend/hibernation, in which case it will be necessary to update the rfkill
67core with the current state is at resume time.
68 69To create an rfkill driver, driver's Kconfig needs to have
70 71 depends on RFKILL || !RFKILL
72 73to ensure the driver cannot be built-in when rfkill is modular. The !RFKILL
74case allows the driver to be built when rfkill is not configured, which which
75case all rfkill API can still be used but will be provided by static inlines
76which compile to almost nothing.
77 78Calling rfkill_set_hw_state() when a state change happens is required from
79rfkill drivers that control devices that can be hard-blocked unless they also
80assign the poll_hw_block() callback (then the rfkill core will poll the
81device). Don't do this unless you cannot get the event in any other way.
82 83 84 855. Userspace support
86 87The recommended userspace interface to use is /dev/rfkill, which is a misc
88character device that allows userspace to obtain and set the state of rfkill
89devices and sets of devices. It also notifies userspace about device addition
90and removal. The API is a simple read/write API that is defined in
91linux/rfkill.h, with one ioctl that allows turning off the deprecated input
92handler in the kernel for the transition period.
93 94Except for the one ioctl, communication with the kernel is done via read()
95and write() of instances of 'struct rfkill_event'. In this structure, the
96soft and hard block are properly separated (unlike sysfs, see below) and
97userspace is able to get a consistent snapshot of all rfkill devices in the
98system. Also, it is possible to switch all rfkill drivers (or all drivers of
99a specified type) into a state which also updates the default state for
100hotplugged devices.
101 102After an application opens /dev/rfkill, it can read the current state of all
103devices. Changes can be either obtained by either polling the descriptor for
104hotplug or state change events or by listening for uevents emitted by the
105rfkill core framework.
106 107Additionally, each rfkill device is registered in sysfs and emits uevents.
108 109rfkill devices issue uevents (with an action of "change"), with the following
110environment variables set:
111 112RFKILL_NAME
113RFKILL_STATE
114RFKILL_TYPE
115 116The contents of these variables corresponds to the "name", "state" and
117"type" sysfs files explained above.
118 119 120For further details consult Documentation/ABI/stable/sysfs-class-rfkill.
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